wk 8 SP Flashcards
functions of motion
mvement attracts attention
movement of an object relative to an observer provides information about the object’s 3D shape
Movement provides information that helps segregate figures from ground and perceptual organisation (common fate)
Movement breaks camouflage
provides with info to directly interact with environment
informs your heading and time to collision, your movement as well as other objects
The ‘correspondence problem’ highlighted by RDKs (Random dot images where we only perceive a moving square when movement between the two is constant)
suggest that
motion detection is direct.
We cannot imagine a visual system matching point for point over time in these displays.
5 ways to make a spot of light move
1) Real movement
2) Apparent movement
3) Induced movement
4) Autokinetic movement
5) Movement aftereffects
Real movement
how is this detected
Light physically moves
-We percieve movement when the eye is stationary, so the image moves across the retina
- when an image moves across the retina, it stimulates a series of receptors
- There are neurons in the visual system that responds best when a stimulus moves in a particular direction
Movement detectors
how do they work + how are they sensitive to direction
neurons are in pairs of neuron and inhibitor in same order and all pairs are wired to one neuron. if light is moving in one direction, it will excite the inhibitor first, not allowing excitation. IF in other direction, light will excite neuron first (won’t get inhibited). If passes over multiple of these pairs, the main neuron will receive successive excitation and will be stimulated
movement detectors are sensitive to
direction
speed,
how are motion detector (neurons?) sensitive to speed
the distance between the two pairs of receptor-inhibitors.
The excitatory signal travels at a speed that is too fast, the two neurons will be stimulated so that the first neurone’s signal does not reach the main neuron at the same time as the second neuron (will reach before). This will lead to a weaker excitatory response.
The distance between the receptors can be altered to be specific to certain speeds.
Bigger seperation of detectors detects ______ motion
faster
Aperture problem means
output of all detectors must be integrated at some stage (Medial Temporal Area)
because one of these motion detectors only sees a very small area.
Where is the information from motion detectors integrated?
MT (middle temporal area)
Threshold for movement detection
depends on
Depends on object and its surroundings,
e.g. with the dot and surroundings (e.g. add vertical lines in space between A & B, lower threshold)
(easier to detect movement when can compare to the lines)
Perception of Velocity
affected by
perception of motion velocity. Affected by surroundings plus size of both the moving object and framework through which it moves
e.g. cat in a big cage has to move faster as a mouse in a small cage to appear to be moving the same speed.
motion detectors Cannot explain movement perception when:
(1) There is no movement on the retina: as when you follow a moving object with your eyes so your eye movements keep the object’s image stationary on fovea
(2) When you perceive no movement when there is movement on retina: - as when you move your eyes to look at different parts of the scene or as you walk through a scene.
Helmholtz outflow theory:
Moving eyes sends signal to eye muscles. Also sends signals to a comparator.
Movement across retina sends signal to comparator
if comparator observes a difference in these signals. do perceive movement
if the two signals cancel each other out, you don’t perceive movement
when tracking a car, eyes move but retinal signal remains stationary. is movement percieved
yes, movement is percieved
when keeping eyes still and object moves across retina
this is a ____ between signals and therefore we perceive _____
difference
movement
(difference in signals in terms of movement)
when we look around the world
what is the comparison between signals of eye movements and of the retina
eye movement and retinal image movement are the same so we perceive no movement
Convincing evidence for Helmholtz outflow theory
- Afterimages move when we move our eyes (Eye muscle movement signal no retinal movement).
- The world moves when we passively wobble our eyes (retinal movement, no eye muscle movement signal).
- Immobilizing eye-ball results in attempted eye- movement leading to apparent movement of world in opposite direction (Eye movement signal, no retinal movement).
Apparent motion
illusion of movement between two lights by flashing one light on and off, waiting between 40 and 200 msec, then flashing another light on and off
e.g. perception of mvmnt in film = series of static images
Less than 30 msec =
Above 30 – 60 msec=
About 60 msec =
About 60 – 200 msec =
Above about 200 msec =
(delay between lights)
no movement, simultaneous.
partial movement
optimum movement
Beta and phi movement
no movement, successive
Beta movement =
phi movement =
While movement appears to occur between the two lights, it is difficult to actually perceive an object moving across the space between them. Phi perceive an object between.
Distance between two lights also affects perception of apparent movement
as the distance increases, what must be increased to keep the perception of movement
(apparent light)
As distance increases, either the time interval or the intensity of the flashes must be increased to maintain the same perception of movement.
induced movement
object isn’t moving, but background is. (or object moving a little bit, background moving a great deal)
e.g. sat on train, think it is moving backwards but actually, train next to you is moving forwards.
Autokinetic movement
light moves by itself. e.g. a light without background (in dark) will appear to move in an erratic path
Motion after effects
if an observer first views a pattern moving in one direction, and then views the spot of light, the spot (and surroundings) will appear to move the opposite direction
how do motion aftereffects work
two neurons sensitive to movement in opposite directions have constant background firing rate.
After prolonged excitation of one neuron, it experiences the inhibitory period where excitation drops below that of other neurons background firing rate.
Therefore we perceive movement in opposite direction
